This invention relates to a power transfer network and, more particularly, to a power transfer network for transferring high frequency energy from a source to an electrode in a sputtering machine.
Sputtering machines and the use of such machines to form thin films of material are now well known in the art. Such machines usually include one or more electrodes which are designed and arranged to function as cathodes or targets and one or more electrodes which are designed and arranged to function as anodes. The electrical energy employed to excite or drive the machine is normally connected to the targets while the anodes are maintained at or near ground potential.
The use of high frequency, say radio frequency, energy to excite or drive the targets is also well known in the art; this type of energy being necessary to achieve sputtering deposition with certain types of target materials and certain types of substrate materials. The high frequency energy is usually between 1 MHz and 40 MHz. In order to achieve maximum power transfer of the energy from the source to the targets it is the common practice to transfer the energy through a network comprising a resonant L-C circuit.
One type of power transfer network that has been employed in the past includes a coil and a capacitor connected in series resonance and coupled between the source and the target. Generally satisfactory performance has been realized with this network when used with relatively small targets sized around 10 square centimeters and exhibiting stray capacitance to ground of no more than around 100 picofarads and excited at typical 13.56 MHz and at power levels necessary to achieve a desired sputtering rate.
One of the attendant problems with this type of power transfer network is that it will not achieve very efficient power transfer when used with a larger sized electrode which exhibits stray, or parasitic, capacitance to ground of around 200 picofarads or higher.
This problem of relatively high capacitance is further compounded as a difficult-to-drive electrode condition by the decrease in the amount of inductive component (for example, coil, etc.) which can practically be used to bring about resonance. This diminished size and nature of the inductive component severely limits the way it is necessarily connected to the source, the sputtering electrode (target) and the tuning capacitor.
Another significant problem with this type of power transfer network is that it will not produce equipotent field distribution around the sputter electrode if the electrode is relatively large in size, that is, has dimensions on the order of 10 cm by 80 cm, or larger. This is due in part to the small physical size and shape of the inductor which serves to limit the position and connection of the coil as a lumped inductance in relation to the sputtering electrode. In addition, the imbalance as brought about by the relatively linear inductance value distributed in the body of the sputter electrode when configured as a relatively large, and particularly as an elongate sputter electrode.
For certain types of applications it is highly desirable to employ electrodes, say targets, of large physical dimensions, which, because of their extended size will exhibit large parasitic, or stray, capacitance to ground. From an economic standpoint it is important that power be transferred to these targets with a high degree of efficiency and from a performance standpoint it is important that the targets be excited with particular uniformity over their surface area.
An example of a sputtering machine in which the targets are driven by RF energy can be found in U.S. Pat. No. 4,014,779 to M. R. Kuehnle and an example of a power transfer network comprising a coil connected in series with a tuning capacitor and coupled between an RF source and target in a sputtering machine can be found in U.S. Pat. No. 4,025,339 to M. R. Kuehnle.